Change date format for email templates

[1v4n4]

Email template variable {{createdAt}} is sent in ISO8601.

It would be better to have user-friendlier format.
If localization is possible, it should be in the language of the widget (actionPage.locale).
If no localization, in some ddmmyyyy (time?) variant.

The relevant code might be here https://github.com/fixthestatusquo/proca-server/blob/main/proca/lib/proca/service/email_merge.ex.

We should have createdAt on this list

proca-server/proca/lib/proca/service/email_merge.ex

Line 7 in c79a8c0

## Variables exposed to template